This invention relates to an article formed by compacting and sintering iron powder to form a matrix structure and infiltrating the matrix structure with copper metal. More particularly, this invention relates to such article wherein the iron powder contains nickel to further enhance mechanical properties.
U.S. Pat. No. 6,551,373, issued to Alcini et al. in 2003, describes a metal article formed of an iron powder matrix and a copper infiltrant. The matrix is formed by compacting and sintering iron powder containing a phosphorus sintering aid. Molten copper is infiltrated into pores within the matrix. The resulting composite exhibits a superior combination of mechanical properties, including elongation and tensile strength. Nevertheless, it is desired to further improve mechanical properties of the copper-infiltrated iron powder article.
In accordance with this invention, a copper-infiltrated iron powder metal article is ma de by a method that includes compacting and sintering a predominately iron powder that contain significant additions of phosphorus and nickel. Phosphorus is present in an amount effective to produce a concentration in the matrix of between 0.1 and 1.2 weight percent. As used herein, concentrations of constituents of the iron matrix are reported based upon the weight of the matrix without infiltrant, whereas the proportion of copper infiltrant is reported based upon the total weight of iron matrix and infiltrant in the product article. The nickel content is between about 1 and 7 weight percent, and preferably between about 1 and 3 weight percent. The method also comprises infiltrating pores within the iron matrix with a copper metal. Infiltration is preferably carried out during sintering, or may be suitably carried out in a subsequent step. The resulting article thus comprises an iron matrix that contains phosphorus and nickel, and copper metal disposed within pores within the matrix. It is found that the addition of nickel to the matrix, in combination with phosphorus in the matrix and copper metal infiltrant, significantly improves mechanical properties, including tensile strength and elongation.
In accordance with the preferred embodiment of this invention, a composite metal article is manufactured by compacting a powder mixture composed of an iron powder, a ferro phosphorus sintering aid and a nickel alloying agent, sintering the compact to form a porous matrix, and infiltrating pores of the matrix with a copper metal.
The powder mixture is preferably a blend composed predominately of iron powder and containing significant amounts of iron phosphorus powder and nickel powder. A preferred iron phosphorus powder contains ferro-phosphorus intermetallic compound. During heating for sintering, the iron phosphorus forms a transient, low melting liquid phase to enhance bonding between iron particles. Upon continued heating, the phosphorus diffuses into the bulk of the iron, causing the liquid phase to solidify and producing iron phosphorus alloy within the matrix. In addition to aiding in sintering of the iron particles, it is believed that the phosphorus aids in wetting of the iron by the molten copper during infiltration, as described in U.S. patent application Ser. No. 09/843,469, incorporated herein by reference. In general, it is desired to add an amount of iron phosphorus powder effective to produce a phosphorus concentration in the matrix between about 0.01 weight percent and 1.2 weight percent, based upon the weight of the iron matrix. A preferred iron phosphorus addition is between 0.4 and 0.9 weight percent phosphorus. While ferro-phosphorus powder is added in the preferred embodiment, phosphorus may be suitably added in other forms, including as a pre-alloy with iron or other metal.
In accordance with the invention, the metal powder also contains nickel, preferably added as nickel powder. In general, a nickel addition of 1 weight percent or more is effective to significantly improve tensile strength while still providing good elongation. Additions greater than about 7 weight percent nickel provide little additional improvement in mechanical properties while adding undesirably to the cost of the product. A preferred range is between about 1.0 and 3.0 weight percent nickel.
In addition to phosphorus and nickel, the iron powder mixture may contain other alloy agents to further enhance desired properties. Such agents may be suitably added as a separate powder or pre-alloyed with the iron, nickel or phosphorus.
By way of an example, in an optional aspect of this invention, molybdenum may be added in the amount up to about 2.0 weight percent, based on the weight of the matrix. When added in combination with nickel, molybdenum is found to further increase tensile strength and to reduce elongation.
It is desired that the mixture contain less than 0.5 weight percent carbon to minimize the substantial effect of carbon on the mechanical properties of iron. Accordingly, in the examples that follow, carbon was limited to less than 0.03 weight percent, based upon the weight of the matrix. The mixture also preferably contains small amounts, typically less than about 2 percent, of fugitive binders and lubricants to facilitate compaction, but which vaporize during sintering and do not affect the product composition or properties.
In preparation for sintering, the powder mixture is filled into a die and compacted to form a green compact. It is an advantage of the invention that the infiltrated product has substantially the size and shape of the iron powder compact, so that the article may be made to near net shape to reduce machining necessary to finish the product. Mechanical properties within preferred ranges may be obtained utilizing a single action press, although the method may be suitably carried out using a double acting press, powder forging or other compaction techniques. The density of the green compact, and thus the volume of pores available for copper infiltration, is determined by the pressure applied to the powder in the die. In general, improved mechanical properties may be obtained for green compacts having a density between about 6.0 and 7.3 grams per cubic centimeter, although densities greater than about 6.8 grams per cubic centimeter are more difficult to compact because of relative hardness of the nickel powder. A preferred compact density is between 6.5 and 6.8 grams per cubic centimeter.
The green compact is heated to sinter the iron powder and form a porous matrix. In general, sintering is carried out in a vacuum at a temperature in the range of about 1100xc2x0 C. and 1300xc2x0 C. for a time sufficient to diffusion bond the iron particles into an integral structure. As the temperature is heated above 1,050xc2x0 C., the iron and phosphorus form a low melting eutectic liquid phase that wets the iron powder surfaces to promote bonding between the particles. Upon continued heating, the phosphorus diffuses into the bulk of the iron, whereupon the liquid phase solidifies. Also during heating, nickel, derived from the nickel powder, diffuses into the iron and thereby forms an alloy that strengthens the matrix.
The sintered product thus comprises an iron matrix that includes pores formed at interstices between particles. In accordance with this invention, a copper metal is infiltrated into the pores of the matrix to produce an infiltrated article having improved mechanical properties. In a preferred embodiment, copper infiltration is carried out concurrent with heating the green iron compact for sintering. For this purpose, metallic copper is arranged in contact with the green compact prior to sintering. During heating above the copper melting temperature of 1083xc2x0 C., the copper melts and is drawn into the compact by capillary action. While not limited to any particular theory, it is believed that the addition of phosphorus and nickel to the iron powder enhances infiltration by the copper, particularly when infiltration is carried out concurrent with sintering. The presence of phosphorus promotes wetting by the copper of the iron surfaces, and thereby improves capillary action by which the copper metal is drawn into the pores. Moreover, the presence of nickel may also promote infiltration by the copper by forming a bronze phase. During infiltration, a portion of the copper, on the order of about 2 weight percent, diffuses into the iron matrix to increase strength and hardenability. The majority of the copper wets iron surfaces with the matrix and forms fillets within notches or other irregularities that would provide sites for stress concentration and thus reduce mechanical proportions. While it is preferred to carry out infiltration concurrent with sintering in a single heating step, infiltration may be suitably carried out in a separate heating step following sintering.
Infiltration may be suitable carried out using high purity copper. Alternately, the copper metal may contain other constituents, including iron, manganese or silicon, that contribute to the mechanical properties of the copper infiltrant. For example, additions of up to about 1 weight percent silicon to the copper phase are found to improve toughness. Additions of manganese and zinc to the copper phase in amounts up to about 1 percent are believed to reduce erosion by the copper infiltrant of the iron matrix. Commercial grade copper commonly also contains zinc. It is believed that zinc vaporizes during infiltration and that residual amounts in the product do not significantly affect mechanical properties. In the preferred embodiment wherein sintering and infiltration are carried out concurrently, one consequence of the presence of additives to the copper metal may be to reduce the melting temperature of the copper metal and thereby initiate infiltration at an earlier stage during heating.
In carrying out infiltration, it is desired to provide an amount of copper for infiltration that is sufficient to achieve a desired improvement in mechanical properties, while minimizing the amount to avoid adding unnecessarily to the cost of the product. For infiltration carried out using capillary action to draw molten copper into the matrix, the amount of infiltrant is less than the available porosity, resulting in porosity in the infiltrated product and a density less than the theoretical maximum. In general, it is believed that an improvement in mechanical properties is obtained with addition of copper of about 2 weight percent or more. Additions greater than about 23 weight percent are difficult to achieve by capillary infiltration. The optimum amount of copper that is readily infiltrated into the matrix which is dependent upon the porosity of the iron matrix, is dependent upon the density of the green compact. For a compact having a density of about 7.0 grams per cubic centimeter, copper addition up to about 15 weight percent copper are readily achieved; whereas a compact density of about 6.8 grams per cubic centimeter have a greater volume of open porosity and readily accommodate up to about 19 percent. Preferred products may contain between about 10 and 16 weight percent copper, based on the total weight of the product.
The resulting product thus comprises a matrix formed predominately of iron and containing additions of phosphorus and nickel. The matrix also contains copper derived from diffusion from the infiltrant phase. The product also includes a copper phase infiltrating pores within the matrix. Copper-infiltrated iron-nickel products formed from compacts having densities between about 6.5 and 7.0 grams per cubic centimeter and infiltrated between about 10 and 16 percent copper in accordance with this invention have been found to have tensile strength greater than about 600 MPa and an elongation between about 4 and 18 percent. Further improvement in mechanical properties is obtained by heat treating the product following sintering and infiltration. For example, annealing the iron matrix at a temperature between about 650xc2x0 C. and 760xc2x0 C. enhances elongation. Also, heating at a temperature between about 200xc2x0 C. and 315xc2x0 C. is effective to relieve stress, particularly within the copper phase, and thereby enhance elongation.
The following examples are indicative of copper-infiltrated iron powder articles made in accordance with this invention.